He turned their waters into blood How grievous this plague was to the Egyptians may be conjectured from the consideration, that the element of water is one of the two great means of supporting life. And the power of God shines forth the brighter, from the fact, that although the land of Egypt is well irrigated, yet the Egyptians were parched with drought amidst abundance of water. It is afterwards said, that frogs were brought forth, [1] and entered even into the chambers of the kings; by which God manifestly evinced that he was the author of the miracle; for although all Egypt swarmed with frogs, the courts of the kings ought to have been exempt from this nuisance. By the term kings, is denoted either the nobles of the realm, or the king's sons, who were brought up in the expectation of the royal power; for at that time, as is well known, one king alone reigned over all Egypt. From this we learn how easily, and as it were by a kind of mockery, God humbles those who pride themselves in the flesh. He did not gather together an army to fight against the Egyptians, nor did he forthwith arm his angels, or thunder out of heaven, but brought forth frogs, which contemptuously trampled upon the pride of that haughty nation, who held in contempt the whole world beside. It would have been no disgrace for them to have been conquered by powerful enemies; but how dishonorable was it to be vanquished by frogs? God thus intended to show that he has no need of powerful hosts to destroy the wicked; for he can do this, as it were in sport, whenever he pleases.

Footnotes

1 - The Hebrew verb for brought forth is srph, sharats, which signifies to multiply exceedingly; and "the noun is used for creeping things, because they procreate in great abundance. It cannot therefore be more fitly translated, as is observed by Hammond, than by swarming." -- Phillips.

See an account of these plagues in Exodus. 6-11. Compare Psalm 78:43-51. This is mostly a mere enumeration of the plagues in the order in which they occurred, but without, of course, the details of the circumstances attending them. There are no circumstances mentioned here which require particular explanation.

He turned their waters into blood,.... With which Egypt abounded; their streams, rivers, ponds, and pools, so that they had no water to drink; a just judgment upon them for shedding the innocent blood of infants, by drowning them in their rivers; this was the first of the ten plagues, Exodus 7:19, with this compare the second and third vials poured out on spiritual Egypt, whereby blood will be given to antichrist, and to the antichristian states, for they are worthy, having shed so much of the blood of the saints, Revelation 16:3.
And slew their fish; which showed that the miracle was real, that the waters were really turned into blood, since the fish could not live in them, as they might if it had been only in appearance; the rivers of Egypt abounded with fish, this was a principal part of their food, and therefore must greatly distress them; see Numbers 11:5.

He deprived them of their favorite "fish," and gave them instead, [Psalm 105:30] out of the water, loathsome "frogs," and (Psalm 105:31) upon their land tormenting "flies" (the dog-fly, according to MAURER) and "lice" (gnats, according to HENGSTENBERG).